AU-KBC RESEARCH CENTRE

            Multicarrier communications of which OFDM (Orthogonal Frequency Division Multiplexing) is a particular type has become attractive recently for providing multimedia services over multipath channels. Also, it can be considered as a possible alternative to single carrier techniques for providing flexible and high data rate communications for applications like wireless multimedia, wireless internet access, future generation mobile communication systems and powerline communication systems. The main advantage is the flexibility it offers in modulation and multiple access.

            OFDM with adaptive modulation can be used for the downlink communication system (base station to the user) where the available bandwidth is divided into independent subchannels. In a typical multiple access scenario, each user has different QoS and bit rate requirements.  The fact that different subcarriers experience different channel conditions for different users in a frequency selective fading environment can be exploited. Hence, an adaptive strategy, which is used to minimize the total transmit power by optimally allocating subcarriers and bits based on the instantaneous channel information is investigated in this thesis.

            We propose a new Adaptive Allocation Algorithm for Multiuser OFDM (MOAAA), which outperforms existing multiple access schemes like OFDM-TDMA and OFDM-FDMA. The experiments have been conducted with reference to the powerline environment with multipath fading, exponential noise spectrum and narrowband interference. We have presented the results obtained by Monte-Carlo simulation using Matlab® under different channel and noise conditions. We have quantified the improvement in terms of overall transmit power and BER. Our new algorithm achieves a significant gain of 3 to 5 dB in SNR over OFDM-FDMA and TDMA. Hence, the proposed algorithm can be used for efficient multiple access for broadband multimedia communications over frequency selective fading environment.